[Illustration: Fig. 3.--Martin’s Universal Microscope. 1782.]
This stage is strictly a micrometer one, having rectangular movements and a fine adjustment, the movements being accomplished by the fine-threaded screws, the milled heads of which are graduated. The mirror, E, is a double one, and can be raised or depressed by rack and pinion; it is also capable of removal, and an apparatus for holding large opaque objects, such as minerals, can be substituted for it. The accessory instruments are very numerous, and amongst the more remarkable may be mentioned a tube, M, containing a speculum, which can take the place of the tube, R, and so form a reflecting microscope. The apparatus for holding animalcules or other live objects, which is represented at S, as well as a plate of glass six inches in diameter, with four concave wells ground in it, can be applied to the stage, so that each well may be brought in succession under the magnifying power. The lenses belonging to this microscope are twenty-four in number; they vary in focal length from four inches to one-tenth of an inch; ten of them are supplied with Lieberkuhns. A small arm, capable of carrying single lenses, can be supplied at T, and when turned over, the stage of the instrument becomes a single microscope; there are four lenses suitable for this purpose, their focal length varying from one-tenth to one-fortieth of an inch. The performance of all the lenses is excellent, and no pains appear to have been spared in their construction. There are numerous other pieces of accessory apparatus, all remarkable for the beauty of their workmanship.[4]
In addition to the movements described by Quekett, the body-tube with its support can be moved in an arc concentrically with the axis of the triangular pillar, on the top of which it is fitted with a worm-wheel and endless-screw mechanism, actuated by the screw-head, T, below. It must therefore be admitted that Martin led the way far beyond his contemporaries, both in the design and the evolution of the microscope. Furthermore, in his “New Elements of Optics,” 1759, he dealt with the principle of achromatism, by the construction of an achromatic telescope.
At a somewhat later period there lived in London a philosophical instrument maker of some repute, George Adams, who published in 1746 a quarto book, entitled “Micrographia Illustrata, or the Knowledge of the Microscope Explained.” This work fairly well describes “the nature, uses, and magnifying powers of microscopes in general, together with full directions how to prepare, apply, examine, and preserve minute objects.” Adams’ book was the first of the kind published in this country, and it contributed in no small degree to the advancement of microscopical science. Adams writes: “We owe the construction of the variable microscope to the ingenuity and generosity of a noble person. The apparatus belonging to it is more convenient, more certain, and more extensive than that of any other at present extant; consequently, the advantage and pleasure attending the observations in viewing objects through it must be as extensive in proportion.” This is believed to apply to Martin’s several microscopes, and that especially constructed for the king, afterwards improved upon by Adams. Another early form of microscope, Wilson Simple Scroll (1746), stamped on the cover of this book, and has thus become familiar to microscopists, was also made by Adams.